Process for case carburizing and heat treating metals



Patented Aug. '4, 1931 UNITED STATES PATENT QOFFICE PAUL V. BROWER, OF KENMORE, NEW YORK, ASSIGNOR TO'CARBIDE 80 CARBON I CHEMICALS CORPORATION, A CORPORATION OF NEW YORK PROCESS FOR CASE CARB'ORIZING AND HEAT TREATING METALS No Drawing.

This invention relates to improvements in the art of carburizing and heat treating metals, and more particularly to the application of a mixture of carbon dioxide and hydrocarbon gas for such purposes.

Attempts to use hydrocarbon gases in an undiluted state as carburizing agents have generally proven unsuccessful because of the attendant excessive deposition of carbon which soon clogs the inlet and outlet of the carburizing furnace and accumulates on the work. As a remedy, it has been proposed to mix various diluents with such hydrocarbons, air, ammonia, carbon monoxide, hydrogen and nitrogen being among .the diluents which have been used with varying degrees of success. However, these diluents have certain inherent properties whichmake them unsuitable in some instances. Air supports the combustion of some hydrocarbons to such a degree that the temperature in the carburizing chamber is increased abovethe desired point, tending to overheat the steel or other metal. Air also produces an'ex' plosive mixture with some proportions of certain gaseous hydrocarbons; and in some instances is also objectionable because it con- .tains a high percentage of nitrogen. Where a good carbide case is desired, it is undesirable to use ammonia as a diluent because a combined nitride and carbide case is produced. The corrosive action of ammonia on various parts of-the equipment is a decided drawback to its use; leaky apparatus leads to shut-downs for repairs. While nitrogen, hydrogen, and carbon monoxide are available in compressed state in portable containers or cylinders, it is difiicult to maintain a uniform gas pressure by means of the gas regulator valve during the discharge of the contents ofthe cylinder due to the great decrease in pressure. I have discovered that carbon dioxide serves excellently as a diluent for hydrocarbon gases adapted for carburizing, as well as for heat treatment such as annealing, and that mixtures of suitable proportions of gaseous hydrocarbons and carbon dioxide gas eliminate many objections encountered in the use of diluents heretofore employed.

'and certain mixtures thereof. However, I find it decidedly advantageous to use and Application filed July 19, 1927. Serial No. 206,830.

The gaseous hydrocarbon constituent of my improved mixture may be propane, propylene, butane, butylene, iso-butane, ethane, ethylene, methane, city gas, natural gas, etc.,

so for the purposes of this invention prefer such gases as propane, propylene,-butane, butylene, iso-butane and certain mixtures thereof, because these are readily liquefiable, that is to say, at temperatures of about F. and pressures under 200 pounds per square inch, so that large volumes thereof may be conveniently stored and shipped in containers such as tank cars and portable cyclinders. I also prefer to use the hydrocarbon gases just mentioned because their uniform carbon content gives dependable and uniform results as compared with city gases and natural gases heretofore extensive- 7O ly used in gas carburizing. I have obtained excellent results with propane as the principal hydrocarbon constituent of the mixture, such propane being commercially available as a liquid under pressure and supplied in containers such as tank cars and portable cylinders at pressures determined by the vapor pressure of propane.

The improved carburizing and heat treating mixture is desirably produced by mixing carbon dioxide in a gaseous state with a gaseous hydrocarbon, such as propane. Since carbon dioxide gas is also readily liquefiable at ordinary temperatures and commercially'supplied in cylinders in a liquid state, I find-it advantageous to deliver the carbon dioxide for my process from a cylinder or other container ofliquefied carbon dioxide. The convenience with which these constituents of my imp'roved mixture may thus be transported and supplied in large volumes at any point of use is of considerable practical importance. Moreover, since the vapor pressures of such liquefied gases are substantially constant, for a given purity and temperature the pressures of the gases delivered from the containers thereof remain practically constant for all volumes of liquefied gases iii-such containers, which is also a decided advantage, inasmuch as attention.

Various well known appliances may be;

used to'mix the carbon dioxide gas and hydrocarhon gas, e. g. propane, in the proper proportions for the particular work at hand, and deliver the mixture at the required pres sure and velocity to the furnace or other work heating chamber in which the carburizing or other treatment is carried on. For example, the constituent gases may be delivered from above the liquid levels of cylinders thereof, through well known pressure reducing and regulating valves to separate inlets in a suitable gas mixer. The mixture, (when intended for carburizing steel or iron articles, may be then delivered to an appropriate carburizing chamber, from which the mixture is discharged when vitiated. An example of a suitable carburizing furnace is disclosed in patent No. 822,460, issued J une 5, 1906. Various types of gas mixers may be used, such as the well known injector mixing nozzle.

The proportion of carbon dioxide gas used in any given mixture depends upon various factors, such as the particular hydrocarbon gas with which it is mixed, the treatment for which the mixture is intended, and the temperature of such treatment. As an example and without intending to limit this invention thereto, I have successfully carburized steel articles with a gaseous mixture containing about 70% carbon dioxide 7 and about 30% propane, by volume in a carburzing chamber at a temperature of approximately 1700 F., with practically no carbon deposition in said chamber nor on the work.

While the furnace action of my improved mixture in producing the superior results disclosed by my investigations is not exactly known to me, it appears that these improve ments are probably due to the formation of carbon monoxide when the carbon dioxide at the elevated temperatures reacts directly with the hydrocarbon gas or combines with the excess carbon thereof, according to the followingv equation The carbon monoxide so formed acts in con]unct1on with the remaining hydrocar- D bon to form a carburized zone with a lower zone of about 1% which is usually the value desired. Excessive carbon contents in the carburized zone give articles different to machine and which are brittle and subject to cracking during subsequent heat treatment operations.

Since a moderate carbon content of 1.0% is obtained at atmospheric pressure and at one pound pressure (gage), pressure carburizing, which increases .the rate of penetration andthe carbon content, can be used without the increase in carbon content ex" cessive e. g. above about 1.2%. Higher car bon content might be obtained at the surface of the work at sufiiciently' high pressures, but would not extend beyond-such a depth as would be largely removed by machining; while with pure hydrocarbons, some city gases, and natural gases, the vhigh carbon zone is so high and so deep that it cannot be removed by machining, thus leaving the finished articles with a brittle surface that is apt to crack during subsequent heat treat ments.

Since the mixture used in the present pro cess is adapted to pressure carburizing, I am able to carburize at temperatures as low as 1550 F. and at pressures above 15 pounds per square inch gage and yet-obtain a rate of case penetration substantially the same as that obtained when employing considerably higher carburizing temperatures. This is an advantage in treating some metals, because such lower temperatures are less apt to distort the work and produce undesirable grain growth. Inasmuch as the liquefied gases which I prefer to use are initially available from their containers at various pressures above atmospheric, I may directly produce the desired mixture at any appropriate pressure, without pressure boosters, blowers or similar extra equipment hereto.- fore required for pressure carburizing. Hence, I may use my improved process and mixture to carburize at temperatures up to about 1850 F. and at super-atmospheric pressures up to about pounds per square inch gage, thereby reducing the carburizing period and the grain growth while also im-.

proving the efficiency of the carburizing operation by producing a greater output for a given equipment and a lower fuel and maintenance expense incident to heating the carburizing furnace.

,For illustrative purposes and with no intention of imposing limitations onthis invention, I give below a specific example of my improved process and the results obtained thereby. The work, such as a number of 0.2% carbon steel articles, was charged in the usual manner into a carburizmg chamber, which was heated to 1700 F. A gaseous mixture comprising about 70% carbon dioxide and about propane, produced as herein described, was passed through the chamber for about three hours while the chamber was held at 1700 F; At the end of this. run, the depth of case was about 0.030 inches. The outer 0.010 inches of the case had a carbon content of about 1.0%; next came a eutectoid region of 0.85

to 0.90% carbon content having a depth of about 0.010 inches; and finally a zone of a depth of 0.010 inches having a carbon conadapted for heat treating, e. g. annealing,

and such operations may be performed with the herein described apparatus, or other apparatus if desired. Generally speaking, the proportions of the constituent-s of a particular mixture required for this purpose depend upon various factors, for example, the composition of the steel or iron being treated. High carbon steels require an atmosphere high in carbon content, whereas low carbon steels should be annealed in an atmosphere of relativelylow carbon content. Hence, in the latter instance the proportion of carbon dioxide gas in the mixture is relatively high while in the former it is somewhat lower; but in most instances the hydrocarbon gas, e. g. propane, is present in such proportions as to produce a substantially neutral atmosphere to prevent decarburization, oxidation or other changes in the steel undergoing annealing, while in some instances the annealing mixture may also have moderate carburizing properties. The particular Way of applying the mixture to the metal in the annealing chamber may vary; sometimes it is circulated slowly around the heated steel, and in'other instances the mixture is used to purge air from said chamber and provide a substantially neutral atmosphere while the heated steel is undergoing treatment. The temperature to which the steel is heated in the heat treating or annealing furnace may be as high as about 1700 F. but depends upon various factors, such as the composition of .the steel and the results desired from the operation.

I claim: a

1. Process which comprises heating a metal article and applying to such article, while heated, a mixture of carbon dioxide and propane. V I 2. Process of case carburizing a steel or non article, comprising applying a mixture of carbon dioxide and propane in the proportion of about by volume of'carbon dioxide and 30% by volume of propane, to the surface of such article, while the article is maintained at a carburizing temperature.

3. Process which comprises heating a metal article and applying to such article a mixture of carbon dioxide and a hydrocarbon gas composed principally of propane.

4. Process which comprises heating a ferrous article to critical temperatures in an atmosphere of mixed gases which-is produced by mixing a hydrocarbon gas composed principally of propane and carbon dioxide, said mixture being capable of reacting at said critical temperatures and forming a mixture which comprises hydrocarbon gas and carbon monoxide without forming deposits of carbon.

5. The process which comprises heating a ferrous article to temperatures between 1550 F. and 17 00 F. in a mixture of carbon dioxide and a hydrocarbon gas at pressures of from 15 to 25 pounds per square inch above atmospheric.

6. Process of case hardening a steel or iron article which comprises heating the article to critical temperatures and applying to the article a mixture of gases composed of about 70% by volume of carbon dioxide and about 30% by volume of a hydrocarbon gas composed principally of propane.

7. Process which comprises heating a metal article and applying to such article a mixture of carbon dioxide and a hydrocarbon gas composed principally of at least one of the following gases: propane, propylene, butane, butylene, iso-butane.

8. Process of case hardening a steel or iron article which comprises heating the article to critical temperatures and applying to the article a mixture of gases composed of about 70% by volume of carbon dioxide and about 30% by volume of a hydrocarbon gas composed principally of at least one of the following gases: propane, propylene,

butane, butylene, iso-butane.

9. Process which comprises heating a metal article to critical temperatures and subjecting the article to gas pressures higher than atmospheric pressures in an atmosphere composed of a mixture of carbon dioxide and a hydrocarbon gas which is composed principally of at least one of the following substances: propane, propylene, butane, butylene, iso-butane.

In testimony whereof, I aflix my signature.

PAUL V. BROWER. 

